Electrolyte



Patented Apr. 25, 1950 2,505,180 nwo'rnoum-z Alexander M. Geortiev andJames J. Campbell, Dayton, Ohio, assixnora to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing.Application April 2, 1947, Serial No. 738,930

2 Claims. (Cl. 175-315) 1 This invention relates to electrolyticcondenscrs and more particularly to improvements in the electrolyte.

The objects of the invention include the provisions of an electrolytewhich can be economically produced from relatively inexpensiveingredients and by a relatively simple method not requiring expensiveequipment and to provide an electrolyte having relatively low viscosity,having greater penetrating properties thereby facilitating theimpregnation of prewound condenser sections, either beiore or afterinsertion in unsealed cans, and permitting the use of thin dense paperin the condenser windings and also to provide an electrolyte havingphysical and chemical stability, thereby permitting the reuse of thesame batch of electrolyte until it is entirely consumed.

The objects of the invention include improvements in electrolyticcondensers by reason of the use of the improved electrolyte.

Further objects of the present invention will be apparent from thefollowing description of methods of manufacture thereof.

The new electrolyte is the resultant of the combination ethylene glycol,boric acid and ammonium borate, the quantity of ethylene glycol beingvery large in proportion to the quantities of the other two ingredients.

Satisfactory results can be obtained by using commercial ammonium boratewhich contains water of crystallization, and hydrous (ortho) borls acidH3130; or anhydrous boric acid B203.

The proportions can be:

Per cent Ammonium borate "about..." 22 to 5 -Hala 3 to 18 Ethyleneglycol 95 to 30 .200? F. Its pH is in the range of 4 to 5.

Another method consists of mixing boric acid, either HaBOs or B203(anhydrous), with ethylene glycol in about the proportions of 5 to 20%boric acid and 95 to 80% ethylene glycol. This mixture is agitated andheated, preferably between 230 and 300 F., for a time sufllcient todissolve the solids and, eventuallyexpel excess water and to obtain acertain resistance, for example, about 200,000 ohms per cubic centimeterat 85 F- It is desirable to obtain a certain high resistance as astarting point for the step which follows. Then enough ammonia gas(usually about 2% to .6% of the total weight of the electrolyte) iscaused to bubble through the cooled composition of boric acid andethylene glycol to reduce the resistivity to the desired value. Theresistivity at F. is in the range of 1000 to 4000 ohms per cubiccentimeter. The exact value depends on the particular voltage rating andapplication of the condenser, 1500 to 3000 ohms per cubic centimeterbeing typical. Its pH is of the order of 4 to 5.

In either case an electrolyte is obtained comprising ethylene glycol andthe reaction products of ethylene glycol with boric acid and ammoniumborate. While the electrolyte has a rathed milk acid reaction, it doesnot attack the aluminum electrodes of the electrolytic condenser and hasexcellent film-forming and filmsustaining properties. The ammonia in comblnation reduces the resistivity and, as a glycol compound of ammoniumborate, provides a filmmaintaining substance.

The second method has the advantages of greater economy in materialsused, saving in time and a more convenient and precise control of thecomposition as to proportions of ingredients, presence of water andelectrochemical characteristics: pH, resistivity, film-forming andfilm-sustaining properties. The resistivity of the electrolyte isprimarily a function of Water content and of ammonia content. For thebest results from the film-forming point of view, the desiredresistivity should be obtained by separately controlling these factors.This separate control is obtained by following the second method whichrequires evaporation of water from the mixture of ethylene glycol andboric acid before ammonia gas is added in order to obtain apredetermined resistance of high value; and then the addition or"ammonia gas to reduce the high resistance to a predetermined moderateresistance. This is preferred to the evaporation which concurrentlyexpels water and ammonia. By using the second method very small portionsof the ingredients are lost in the course of evaporation. Sinceevaporation takes place prior to the bubbling of ammonia. none of theammonia will be lost.

Resume of important novel features of the electrolyte Due to the greatpercentage of solvent in this electrolyte and its extreme fluidity (itis about as fluid as the pure ethylene glycol alone) it provides auniform and continuous wetting and coverage of the aluminum electrodes.The resulting advantages are: rapid and thorough impregnation ofprewound sections, either before or after placing in unsealed cans,possibility of using thin, dense paper in winding the sections, whichcontributes to the compactness of the condensers, uniformity andemciency in the operation of the total electrode area, since ahomogeneous electrolyte is distributed all over the aluminum surfaces.Easy diffusion of the electrolyte throughout the section, which tends toequalize the temperature, averts the formation of hot spots in thewinding and thus helps it to withstand electrical overloading. The highpercentage of ethylene glycol, which is a plasticizer, prevents thedrying out of the condenser in case of overheating and exposure to theair. Drying out is the common weakness of condensers made with theconventional viscous or semi-solid glycol electrolytes. The formation oflumps of salt, as ordinarily found in other electrolytes made of thesame ingredients, is minimized or avoided.

The elimination of waste is even more pronounced in the course ofimpregnation of condenser sections, because the electrolyte does notcake, i. e. does not form insoluble precipitates, and can be reusedindefinitely, until completely consumed. Any precipitate which mayeventually be formed on cooling is redissolved on reheating. Otherelectrolytes made of the same ingredients and heretofore used have astrong tendency to cake and become unsuited for reuse. Large proportionsof the electrolyte are therefore discarded from time to time, resultingin anpreciable material losses. It has been experienced heretofore that,after repeated heating of the electrolyte, cakes or substantially solidlumps would be formed. Such caking would take place when several batchesof condenser sections are impregnated in succession in the same bath.Such caking rendered the electrolyte unsuitable because it interferredwith the proper impregnation of the condenser sections. It necessitatedthe discarding of the electrolyte batch thus causing waste of materialand increasing the cost of production. The present electrolyte possessesphysical and chemical stability thereby enabling the same batch to bereused until entirely consumed. Waste of materials is avoided with theresult that the cost of production is reduced.

The outstanding penetrating properties of the present electrolyte permitthe impregnation of prewound sections without resorting to the use ofcomplicated, costly equipment. They also markedly shorten the timerequired for impregnation. Due to these penetrating properties it isalso practicable to impregnate the condenser sections after they havebeen inserted but not sealed in their containers. This procedure results in an appreciable saving in labor, in the uniform absorption ofelectrolyte and expansion (swelling) of the sections in the course ofimpregnation and in the elimination of the handling of impregnatedsections, which may entail their contamination. This method ofimpregnation in containers also provides an automatic, firm and lastingpositioning of the section in the can, thus making wedging (a slow andcostly operation) unnecessary.

Characteristics of the condensers impregnated with the newelectrolyte 1. Low leakage current, low power factor, high capacity perunit area of foil.

2. Good breakdown characteristics across the line.

3. Effective and uniform cooling while in operation, due to thediffusion of the fluid electro lyte. Avoidance of hot spots.

4. Resistance to drying out when subjected to prolonged heating (whencurrent allowed to flow in condenser for a long time), and or ex posedto the air. This is due to the great proportion of ethylene glycolversus solid constit uents (boric acid, ammonium borate) used in thepreparation of the electrolyte.

5. Compactness, because thin, dense paper spacers can be used in windingof condenser section.

6. Easier and better assembling of condensers due to practicability ofimpregnating sections after their insertion in cans when this fluidelectrolyte is used.

While the form of embodiment of the present invention as hereindisclosed, constitutes a preferred form, it is to be understood thatother forms might be adopted, all coming within the scope of the claimswhich follow.

What is claimed is as follows:

1. The method of making a low viscosity, substantially anhydrouselectrolyte for electrolytic condensers, which comprises; mixing 5% to20% boric acid with 95 to ethylene glycol. by weight, heating themixture at a temperature between 230 F. to 300 F. for a time sufficientto completely dissolve the solids and to substantially eliminate water,cooling the reaction mixture, and then passing ammonia gas therethroughin quantities suflicient to bring the resistivity of the solution at F.to within the range of 1000 to 4000 ohms per cubic centimeter and forcontrolling the pH value of the solution to a figure of between 4 and 5,whereby a stable, fluid electrolyte is formed.

2. A low viscosity, substantially anhydrous and stable electrolyte foruse in electrolytic condensers, consisting of an anhydrous, unsaturatedsolution of the reaction mixture of boric acid and ethylene glycolwherein the boric acid varies between 5% and 20%, which solution hasbeen ammoniated by passing ammonia gas therethrough in quantitiessufficient to produce a resistivity within the electrolyte of from 1000to 4000 ohms at 85 F. a viscosity at 200 F. of from 1 to 1 as comparedto the viscosity of water at 60 F. and an acidity having a pH of from 4to 5.

ALEXANDER M. GEORGIEV. JAMES J. CAMPBELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,815,768 Georgiev July 21, 19311,891,207 Ruben Dec. 13, 1932 1,909,506 Ruben May 16, 1933 1,924,711Edenburg Aug. 29, 1933 2,031,128 Raines Feb. 18, 1936 2,062,543 WatermanDec. 1, 1936 2,196,057 Clark Apr. 2, 1940

